Temperature compensating fluid velocity sensing apparatus

Abstract

Apparatus are disclosed for sensing the velocity of a fluid stream, as may be flowing in a duct, pipe, or the like, which fluid stream exhibits variations in its temperature under different operating conditions. The apparatus includes a first probe which is exposed to the fluid stream and includes first temperature sensing means for sensing the temperature of the fluid stream. A second probe is provided which is disposed within the fluid stream and includes at least a portion constructed of a thermally conductive material. Heater means are disposed in thermally conductive relationship with said thermally conductive portion, and second temperature indicative of the temperature of said heater means. Third temperature sensing means are provided for sensing the temperature of the thermally conductive portion of the second probe at a point removed from the heater means, which temperature is a function of the heater means temperature, the fluid temperature, and the velocity of the fluid flowing thereover. In order to compensate for variations in the fluid temperature, heater control circuit means are provided for receiving temperature signals from the first and second temperature sensing means and for controlling the heat output of the heater means so as to maintain substantially constant the temperature difference between the fluid and heater means. Velocity output circuit means are provided for receiving the fluid temperature signal from the first temperature sensing means and the second probe temperature signal from the third temperature sensing means and for producing a signal related to the differences between said temperatures, which signal is related to the velocity of the fluid stream.

We claim: 
     
       1. Apparatus for sensing the velocity of a variable temperature fluid stream comprising a. first probe means including first temperature sensing means for sensing the temperature of said fluid and producing a first signal related thereto;   b. second probe means including at least a portion constructed of thermally conductive material to be disposed in said fluid stream, said probe means including i. heater means disposed in thermally conductive relationship with the thermally conductive portion of said second probe means;   ii. second temperature sensing means for sensing a temperature indicative of the temperature of said heater means and for producing a second signal related thereto; and   iii. third temperature sensing means for sensing the temperature of the thermally conductive portion of said second probe means at a point removed from said heater means and for producing a third signal related thereto;     c. heater control circuit means for receiving said first and second signals and for controlling the heat output of said heater means so as to maintain substantially constant the temperature difference betweeen said fluid stream and said heater means; and   d. velocity output circuit means for receiving said first and third signals and for producing a fourth signal related to the difference between the temperature of said fluid and the temperature of said second probe means at said point removed from said heater means, said fourth signal being related to the velocity of said fluid stream.   
     
     
       2. The apparatus of claim 1 wherein said first and second probe means each comprise an elongated, tubular metal enclosure. 
     
     
       3. The apparatus of claim 1 wherein said first, second, and third temperature sensing means each comprise a thermistor. 
     
     
       4. The apparatus of claim 1 wherein said second temperature sensing means is disposed immediately adjacent said heater means so as to directly sense its temperature. 
     
     
       5. The apparatus of claim 1 wherein said second probe means comprise an elongated, tubular metal enclosure having a closed end, said heater means being disposed within and in thermally conductive relationship with said enclosure at a point along its length removed from said closed end, and said third temperature sensing means being disposed within said enclosure at a point removed from said heater means adjacent said closed end. 
     
     
       6. The apparatus of claim 5 wherein said second temperature sensing means is disposed within said enclosure at a point immediately adjacent said heater means so as to directly sense its temperature. 
     
     
       7. The apparatus of claim 1 wherein said first probe means comprise an elongated tubular metal enclosure having a closed end, said first temperature sensing means being disposed within said enclosure at a point adjacent said closed end. 
     
     
       8. The apparatus of claims 5, 6, or 7 wherein said first, second, and third temperature sensing means each comprise a thermistor. 
     
     
       9. The apparatus of claim 1 wherein said first probe means include a least a portion constructed of thermally conductive material to be disposed in said fluid stream, said first temperature sensing means being disposed in thermally conductive relationship with said portion so as to sense the temperature of said fluid passing thereover. 
     
     
       10. The apparatus of claim 1 wherein said heater control circuit means include a. means for combining said first and second signals so as to produce a fifth signal related to the difference in temperature between said fluid and said heater means;   b. comparator means for receiving said fifth signal and comparing same with a reference signal related to a predetermined difference in temperature between said fluid and said heater means, said comparator means being operative to produce an output signal when the difference in temperature between said fluid and said heater means falls below said predetermined difference; and   c. switch means responsive to said output signal for energizing said heater means so as to increase its temperature.   
     
     
       11. The apparatus of claim 1 or 10 wherein said velocity output circuit means comprise means for combining said first and third signals so as to produce said fourth signal related to the difference in temperature between said fluid and said second probe means at said point removed from said heater means, said fourth signal being related to the velocity of said fluid stream. 
     
     
       12. The apparatus of claim 11 wherein said velocity output circuit means further coimprise amplifier means for increasing the amplitude of said fourth signal. 
     
     
       13. The apparatus of claim 11 wherein said first, second, third, and fourth signals comprise voltage signals.